Local area network utilizing macro and micro diversity techniques in receiving signals from cell sites

ABSTRACT

A communications network comprises two or more cell sites for communication with wireless terminals. At least one of the cell sites has multiple receive antennas. A central site has one or more interface controllers and a switch system through which the controllers are connected to the cell sites. For each controller in communication with a wireless terminal, a cell site is selected for reception of signals from the terminal, and for each selected cell site having more than one receive antenna, an antenna within the site is selected for reception from the terminal.

FIELD OF THE INVENTION

This invention relates to diversity in wireless networks, and inparticular to implementations of macro and micro diversity at adistributed access point in a wireless LAN. The network may implement arange of wireless protocols such as IEEE 802.11a and 802.11b forterminals communicating through the access point.

BACKGROUND TO THE INVENTION

Wireless LANs (local area networks) are emerging as importantinfrastructure for a wide range of commercial and domestic premises.They enable mobility of wireless devices about the premises and aregenerally more flexible and lower cost than networks with equivalentwired connections. However, a large number of wireless access points maybe required to properly serve the coverage volume of a particularnetwork, and different mobile devices may require service within thevolume using different wireless protocols. This increases the number ofwireless interface controllers with RF (radio frequency) transceiversthat are required by the network, and therefore increases its cost.Distributed access points having a number of relatively simple cells orPOPs (points of presence) for transmission and reception of RF signalsare therefore under development. Each access point has a central serverwith a set of transceivers that are typically connected to the POPs byoptical fibres, coaxial cables or the like, through a bridge or switch.

Diversity techniques can improve wireless communication by selecting onechannel, or combining a subset of channels, from a range of decorrelatedchannels that may exist between a transmitter and a receiver. A range ofdifferent algorithms exist for selecting or combining signals from amongthe available channels. In spatial diversity each channel is a physicalpath between a transmitting antenna and a receiving antenna, withselection of the most suitable receiving antenna according to analysisof the respective signal. Macro diversity counters large scale andgenerally static spatial variations between the receiving antennae suchas shadowing. Micro diversity counters relatively small scale and oftentime varying effects such as multipath fading. Both macro and microspatial effects can be important in wireless LANs with distributedaccess points.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide for the use ofspatial diversity techniques in wireless LANs having distributed accesspoints, or at least to provide an alternative to existing diversitysystems in wireless networks.

In one aspect the invention may broadly be said to consist in acommunications network comprising: two or more cell sites forcommunication with wireless terminals, at least one of the cell siteshaving multiple receive antennas; and a central site having one or moreinterface controllers and a switch system through which the controllersare connected to the cell sites; wherein for each controller incommunication with a wireless terminal, a cell site is selected forreception of signals from the terminal, and for each selected cell sitehaving more than one receive antenna, an antenna within the site isselected for reception from the terminal.

Preferably the network further comprises: a cell selector in the centralsite that uses a diversity technique to select cell sites for receptionfrom the wireless terminals and connects the selected sites torespective controllers through the switch. Preferably the networkfurther comprises: an antenna selector in each controller that uses adiversity technique to select an antenna within each cell site havingmultiple receive antennas. Preferably the interface controllers includetransceivers that transmit and receive RF signals according torespective protocols that are used by the wireless terminals, andpreferably the central site is connected to at least some of the cellsites via optical fibres.

The invention may also be said to consist in any alternative combinationof features that are suggested in this specification of the drawings.All equivalents of these features are deemed to be included whether ornot explicitly set out.

LIST OF FIGURES

Preferred embodiments of the invention will be described with respect tothe accompanying figures, of which:

FIG. 1 shows a prior art wireless network with a distributed accesspoint,

FIG. 2 shows how macro diversity may be implemented in a distributedaccess point,

FIG. 3 shows how micro diversity may be implemented in a distributedaccess point,

FIG. 4 shows how both macro and micro diversity may be implemented in adistributed access point,

FIG. 5 shows how macro and micro diversity may be alternativelyimplemented in a distributed access point, and

FIG. 6 gives detail of the macro diversity selection in FIG. 5.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings it will be appreciated that the invention canbe implemented in a range of wireless networks in a range of differentways. The embodiments described here are given by way of example only.

FIG. 1 schematically shows part of a network serving a number ofwireless terminals WT1-4 through a distributed access point. The accesspoint includes a server 10 connected to a wired network 11 and to anumber of points of presence POP1-3, being three in this example, eachhaving RF transmit and receive antennas. Each POP represents a smallcell site for radio communication. The server contains one or morecontrollers, commonly called NICs (network interface cards) that carryout various functions including conversion of signals between protocolsand carriers used on the wired and wireless parts of the network. Thecontrollers are connected to the POPs through a switch 12 that enablesany one of the NICs to transmit and receive RF or IF signals through anyone of the POPs. Each POP has a connection 13 to the switch, by way ofoptical fibre in this example, with RF signals being modulated anddemodulated onto and from optical signals at each end of a fibre. Aseries of ports are provided for each of the POPs respectively in anoptoelectronic module 14. Connections may also be made by a range ofother means such as coaxial cable. The wireless terminals WT1-4 maycommunicate with each other, and with fixed terminals 15 or otherwireless and fixed terminals in the network through the access point.

FIG. 2 schematically indicates how macro diversity techniques may beimplemented among POPs at the distributed access point in FIG. 1. Server10 includes two interface controllers NIC1-2, provided as cards, thatenable communication with devices using any of a range of wirelessprotocols such as IEEE 802.11a and 802.11b, for example. The two NICs, aprocessor 20, memory 21 and network port 22 in the server are connectedby address, data and control buses, shown in simple form for clarity.Processor 20 carries out a range of general functions for the accesspoint, including frequency planning, power control, diagnostics andnetwork management via SNMP (simple network management protocol) forexample. Switch 12 preferably includes separate transmit and receiveswitching components 23 and 24 respectively, to reduce cross talk whentransferring transmit and receive signals between the NICs and the POPs.Both switch components are set by a controller 25. Switch connectionshave been shown in simple form for NIC1 only. Opto-electronic module 14includes a series of ports 26 for fibre optic connection to the POPs,each having an optical transmitter and an optical receiver, typically.lasers L1-3 and photodiodes P1-3 respectively. Outputs from the transmitswitching component 23 are connected to the lasers L1-3.

Macro diversity in FIG. 2 is implemented by way of a selection system 27connected between the opto-electronic module 14 and the receiveswitching component 24 of the switch 12. Outputs from the photodiodesP1-3 are connected to inputs of the receive switching component 24, withportions of the signals being passed through filters F1-3 respectivelyin the selection system. Output from each of the filters is connected toa detector 28. The signal received from a POP may contain individualsignals representing separate communications on several channels inseveral bands. For example, a POP that is constructed to enable IEEE802.11a and b will potentially receive signals on several channels ineach of the 5.2 GHz and 2.4 GHz bands that have been defined for thosestandards. The filters F1-3 are therefore preferably narrow-bandper-channel filters and the detector 28 preferably has a separate powerdetector for each possible channel for each POP. Power or signal levelsare preferably detected near the channel centres only, or patternmatched across multiple channels, to improve rejection of power fromneighbouring channels. Various known diversity algorithms can beimplemented by the selection system 27 for analysis of the signals fromeach POP, such as signal power, channel delay spread, channel matrixeigenvalue spread and preamble soft error magnitude. Output from theselection system 27 is used by the switch controller 25 to set pathwaysthrough the receive switching component, so that each NIC in the server10 receives an optimum signal from a selected POP.

FIG. 3 schematically indicates how micro diversity techniques may beimplemented in POPs at the distributed access point in FIG. 1. Many ofthe elements in this figure are the same or similar to those of FIG. 2.Switch connections have been shown in simple form for NIC2 only. EachPOP now has more than one receive antenna and is able to provide two ormore versions of the same signal to the server 10 through the switch 12.In this example, each POP has two receive antennas that provide signalsto the opto-electronic module 14 along separate optical fibres. Eachport 26 in the module 14 now includes an optical transmitter such as alaser L1-3, and two optical receivers such as a photodiode P1-3 andP1′-3′. Separate receive switching components 30, 31 are preferablyprovided in the switch 12, corresponding to the number of signals fromeach POP. The transmit and receive switching components are set by thecontroller 25. A diversity selection system 33 in each of the interfacecontrollers NIC1-2 selects from the received signals from a particularPOP. Both of the received signals from a particular POP are delivered toa particular NIC by the switch controller generally according to macrodiversity techniques that may also be implemented. Various knowndiversity algorithms can be implemented by the selection systems 33 foranalysis of the signals from each POP as mentioned above.

FIG. 4 shows schematically how a distributed access point combining bothmacro and micro diversity techniques may be implemented. Many of theelements common to the preceding figures have been omitted, while someextra detail has been included, and only a single NIC 40 has been shown.In this example, there are eight POPs 41 each with a transmit antenna 42and two receive antennas 43, 44 enabling selection from eight channelsfor macro diversity and two channels for micro diversity. Each antennamay operate in multiple bands, covering both of the bands prescribed forIEEE 802.11 a and b, for example. Each POP is connected by opticalfibres to an opto-electronic port 26 having a laser L for opticaltransmission and two photodiodes P, P′ for optical reception. In thisexample, the outgoing signal to each POP shares an optical fibre 45 withone of the incoming signals from the POP, while a separate fibre 46 isprovided for the other incoming signal. The POPs may also containlasers, photodiodes, amplifiers, filters, frequency converters andoptical multiplexers as required. The NIC carries out both macro andmicro diversity selection, with all of the signals received from all ofthe POPs capable of being presented to the selection system 47. Somespecific components of the NIC including an I/O port for connection tothe server, a MAC processor for analysing packets received from wired orwireless terminals, a baseband modem for conversion of digital signalsto and from quadrature form, and an RF or IF stage for modulation anddemodulation of the quadrature signals onto high frequency carriers arealso shown. Specific transmit and receive connections are also shown inthe switch, again by way of example.

FIG. 5 shows an alternative implementation of macro and micro diversityin a distributed access point. Many of the elements common to thepreceding figures have been omitted for simplicity, while some extradetail has been included. In this example there are again eight POPseach with two receive antennas, enabling macro and micro diversityselections from eight and two channels respectively. Four NICs 50 havenow been shown with specific receive switching connections 51, andopto-electronic ports 52. Only a few of the total number of receiveconnections in the switch have been shown, while the transmitconnections have been omitted altogether. The two receive switchcomponents preferred for transfer of signals from each pair of receiveantennas in the POPs are shown in overlay form. Macro diversityselection is carried out on the POP side of the switch by filters andpower detectors in block 27 as in FIG. 2, with adjustment of the switchbeing carried out by control 25. Micro diversity selection is carriedout by the NICs as in FIG. 3. Alternatively, both macro and microdiversity selection may be carried out on the POP side of the switch.

FIG. 6 gives some detail of the diversity selection system 27 in FIGS. 2and 5. A pair of signals is input to the selection system from a POPhaving two receive antennas. The signals are received along an opticalfibre by an O/E card 60 containing suitable opto-electronic components,typically photodiodes. Each signal from an antenna may contain componentsignals from a range of wireless terminals using a range of differentprotocols. In this case, the signals are broadly filtered 61 accordingto two prescribed bands, such as the 2.4 GHz and 5.1 GHz bands of theIEEE 802.11 standards. A portion of the signal in each band is thenfiltered by a respective bank of narrow band filters 62 covering each ofthe available channels. The power in each channel is determined andoutput as data to a local processor 63, generally part of the switchcontrol 25. A control signal from the main processor 20 in the accessserver 10 in FIG. 2 is also usually received. Output from the localprocessor is used to determine the switch settings 51 in FIG. 5. Thepair of signals are then passed through buffers 64. Diversityarrangements implemented according to the invention are able to achieveselection and switching on practical time scales. The selection processshould generally take place as close as possible to the switchingmechanism to reduce any delay in operating the switch. It is preferredthat selection by macro diversity take place prior to switching and thatselection by micro diversity take place after the switch. However, itwill be appreciated that various combinations of diversity selection andswitching will be appropriate in different networks.

1. A communications network comprising: two or more points of presencefor communication with wireless terminals, each point of presencerepresents a cell site, each point of presence having multiple receiveantennas which provide diversity reception of wireless signals at thepoint of presence; a central site having one or more controllers, theone or more controllers comprise a selection system, the selectionsystem carries out macro-diversity selection using a cell selector andmicro-diversity selection using an antenna selector; and a switch systemthrough which receive signals from each of the multiple receive antennasof each point of presence are connected to the selection system, whereinbased on analysis, at the selection system at the central site, of thereceive signals from each of the multiple receive antennas of each pointof presence: (a) the cell selector performs the macro-diversityselection to counter macro spatial effects in the communicationsnetwork, and selects one of the points of presence from the two or morepoints of presence for reception from a particular wireless terminal,and (b) the antenna selector performs the micro-diversity selection tocounter micro spatial effects in the communications network, and selectsone of the receive antennas of the multiple receive antennas of theselected point of presence, such that the antenna selector selects amicro-diversity channel from each point of presence from among at leasttwo micro-diversity channels from each point of presence, and the cellselector selects a macro-diversity channel from among the selectedmicro-diversity channels.
 2. A communications network according to claim1, wherein: the switch system presents all receive signals from themultiple receive antennas at each point of presence to the selectionsystem at the central site for use by the antenna selector in selectingthe micro-diversity channel from each point of presence from among theat least two micro-diversity channels from each point of presence, andfor use by the cell selector in selecting the macro-diversity channelfrom among the selected micro-diversity channels.
 3. A communicationsnetwork according to claim 1 wherein the one or more controllers includetransceivers that transmit and receive RF signals according to differentrespective wireless LAN protocols that are used by different wirelessterminals.
 4. A communications network according to claim 1 wherein thecentral site is connected to the two or more points of presence viaoptical fibers, and each point of presence comprises an opticaltransmitter and an optical receiver.
 5. A communications networkaccording to claim 1, wherein the multiple receive antennas of eachpoint of presence includes first and second receive antennas, thecommunications network further comprising, for each point of presence: afirst electric-to-optical converter associated with the first receiveantenna, and a second electric-to-optical converter associated with thesecond receive antenna; an optoelectronic port having at least first andsecond optical receivers; a first optical fiber coupled between thefirst optical receiver and the first electric-to-optical converter tocarry a receive signal of the first receive antenna; and a secondoptical fiber coupled between the second optical receiver and the secondelectric-to-optical converter to carry a receive signal of the secondreceive antenna.
 6. A communications network according to claim 5,wherein, for each point of presence, the antenna selector selects one ofthe receive antennas by selecting a signal from a set of signals whichincludes signals of the first and second optical receivers.
 7. Acommunications network according to claim 5, further comprising, foreach point of presence: a transmit antenna; an optical-to-electricconverter associated with the transmit antenna; and an opticaltransmitter associated with the optoelectronic port; wherein the opticaltransmitter is coupled to the optical-to-electric converter of thetransmit antenna to carry a transmit signal of the transmit antenna bysharing at least part of the first optical fiber with the receive signalof the first receive antenna.
 8. A communications network according toclaim 1, wherein: signals of the multiple receive antennas are receivedat the antenna selector; and the antenna selector selects the onereceive antenna of the multiple receive antennas by selecting one of thesignals of the multiple receive antennas and passing the selected one ofthe signals of the multiple receive antennas to the cell selector.
 9. Acommunications network according to claim 1, wherein: the one or morecontrollers are provided in a network interface card.
 10. Acommunications network according to claim 9, wherein: the networkinterface card comprises a MAC processor for analyzing packets receivedfrom each point of presence according to a wireless LAN protocol.
 11. Acommunications network according to claim 10, wherein: the wireless LANprotocol is IEEE 802.11.
 12. A communications network according to claim1, further comprising: at least one network interface card, the at leastone network interface card comprises the one or more controllers, abaseband modem for conversion of digital signals to and from quadratureform, a stage for modulation and demodulation of quadrature signals, andan input/output port for connection to a server.
 13. A communicationsnetwork according to claim 1, wherein: for each point of presence, themultiple receive antennas include first and second receive antennaswhich provide diversity reception of wireless signals at the point ofpresence in a first frequency band according to a first wireless LANprotocol, and third and fourth receive antennas which provide diversityreception of wireless signals at the point of presence in a secondfrequency band according to a second wireless LAN protocol.
 14. Acommunications network according to claim 13, wherein: the firstwireless LAN protocol is IEEE 802.11a and the second wireless LANprotocol is IEEE 802.11b.
 15. A communications network according toclaim 13, wherein: the first frequency band is a 5.2 GHz band, and thesecond frequency band is a 2.4 GHz band.
 16. A communications networkaccording to claim 13, wherein: signals received by the first and thirdreceive antennas are carried by a first optical fiber to anopto-electronic port; and signals received by the second and fourthreceive antennas are carried by a second optical fiber to theopto-electronic port.